Within the twelve page document they discuss flaws in various APIs and other certificate checking software, and for libcurl they say:

Internally, it uses OpenSSL to verify the chain of trust and verifies the hostname itself. This functionality is controlled by parameters CURLOPT_SSL_VERIFYPEER (default value: true) and CURLOPT_SSL_VERIFYHOST (default value: 2). This interface is almost perversely bad. The VERIFYPEER parameter is a boolean, while a similar-looking VERIFYHOST parameter is an integer.

(The fact that libcurl supports no less than nine(!) different SSL library backends seems to have been ignored but is irrelevant.)

The final part is their focus. It is an integer option but it looks like it could be similar to the VERIFYPEER option which could be considered a boolean option – but note that there is no boolean options at all in libcurl, those are all “long” values. They go on to explain:

Well-intentioned developers not only routinely misunderstand these parameters, but often set CURLOPT_SSL_VERIFY HOST to TRUE, thereby changing it to 1 and thus accidentally isabling hostname verification with disastrous consequences

They back up their claim with some snippets from PHP programs showing wrong use in chapter 7.

What did the authors do to try to fix the problem before posting rude comments in a report? Nothing. At. All. They could’ve emailed, tweeted or posted a bug report or patch but none of that happened.

They also only post examples of the bad use made by PHP code. The PHP code uses the PHP/CURL binding and a change could easily be done in the PHP binding. I don’t know PHP internals, but perhaps the option could be made to not accept a boolean value instead of a numerical there.

We’re now discussing this topic on the libcurl mailing list. If you have ideas or suggestions or just comments, feel free to join in!

I will also call the majority of all these suddenly appearing complainers on this API to be mostlyÂ hypocritesÂ since the API has been established and working like this for over a 10 (ten!) years and not a single person has objected to it before. Joining up on the “bandwagon” now and calling the API stupid or silly is… well, I’d call it “non-intelligent behavior”. In libcurl we take a stable and solid API and ABI very seriously. We simply do not break API nor ABI unless forced brutally into a corner we can’t escape otherwise. ThereforeÂ we have kept this API to keep existing applications functional.

Second update: we shipped libcurl 7.28.1 on November 20 2012, and it no longer accepts the value 1 to VERIFYHOST, but will instead cause curl_easy_setopt() return an error and use the default value (which is 2). This will prevent applications to accidentally be insecure due to use of 1.